In recent years, driving power for electronic circuits has been significantly reduced, and it has become possible to drive various electronic parts such as sensors with a weak power (of a μW order). Expected uses of sensors include application to energy harvesting elements as stand-alone power systems capable of generating and consuming power instantly. Among such energy harvesting elements, solar cells (which are a kind of photoelectric conversion elements) are drawing attention as elements capable of generating power at anywhere there is light.
Among such solar cells, dye-sensitized solar cells proposed by Graetzel et al. from Swiss Federal Institute of Technology in Lausanne have been reported to have high photoelectric conversion characteristics greater than or equal to photoelectric conversion characteristics of amorphous silicon solar cells under weak room light (see, e.g., Non-patent document 1). Room light of, for example, LED lights and fluorescent lamps typically has illuminance of about from 200 Lux through 1,000 Lux and is by far weaker than direct sunlight (about 100,000 Lux). Structures of the solar cells are formed of porous metal oxide semiconductors on transparent conductive glass substrates; dyes adsorbed to surfaces of the porous metal oxide semiconductors; electrolytes containing redox couples; and counter electrodes. Graetzel et al. have remarkably improved photoelectric conversion efficiencies by using porous materials as the electrodes formed of the metal oxide semiconductors such as titanium oxide to increase surface areas and by monomolecularly adsorbing ruthenium complexes as the dyes (see, e.g., Patent document 1 and Non-patent documents 2 and 3).
Dye-sensitized solar cells are typically produced using electrolytic solutions, but have not been put into practical use yet because of problems of volatilization or leak of the liquids. Meanwhile, as development aiming at practical use, the following solid dye-sensitized solar cells using solid materials in place of electrolytic solutions have been reported.
Hole transport materials are mainly used as the solid materials. Materials that have behaviors like p-type semiconductors and that are capable of receiving holes from dyes can replace electrolytic solutions.
1) Solid dye-sensitized solar cells using inorganic semiconductors (see, e.g., Non-patent document 4)
2) Solid dye-sensitized solar cells using low-molecular-weight organic hole transport materials (see, e.g., Patent document 2 and Non-patent documents 5 and 6)
3) Solid dye-sensitized solar cells using conductive polymers (see, e.g., Patent document 3 and Non-patent document 7)
In solid dye-sensitized solar cells having structures in which porous metal oxide semiconductor layers are provided directly on transparent conductive glass substrates, hole transport materials may intrude through voids in the porous layers and contact surfaces of the transparent conductive glass substrates. This has been known to cause recombination between holes in the hole transport materials and electrons in the surfaces of the transparent conductive glass substrates (i.e., so-called back electron transfer), leading to a fall in electric power.
As compensation for this disadvantage, methods for forming hole blocking layers formed of metal oxides on transparent conductive glass substrates have been reported, and for example, the following reports have been given.
Patent document 4 discloses that an oxide film formed of niobium oxide is interposed between a transparent conductive film and a semiconductor particle layer, and that a film thickness of the oxide film is set to a predetermined value. This is an attempt to improve photoelectric conversion efficiency, because back electron transfer from the transparent conductive film to an electrolytic solution can be prevented without electron transfer to the transparent conductive film being disturbed.
Patent document 5 is directed to a dye-sensitized solar cell including a photocatalytic film on a transparent electrode side, and discloses a method for forming a buffer layer to be disposed between the transparent electrode and the photocatalytic film. A mixture solution obtained by dissolving a metal alkoxide, which is a precursor of the photocatalyst, in an alcohol liquid is coated on a surface of the transparent electrode through a liquid spraying nozzle, and immediately after this, superheated steam is sprayed through a steam spraying nozzle for firing, to form a buffer layer.
Furthermore, as concerns formation of a metal oxide film, a metal oxide film including two or more kinds of metal atoms has been reported as follows. Patent document 6 discloses that a step of performing reactive sputter by an oxygen gas using a first target formed of metal Ti and a step of performing sputter using a second target formed of niobium pentoxide are performed simultaneously to form a transparent conductive film formed of titanium oxide doped with niobium on a substrate.
In the structures of the solid dye-sensitized solar cells, hole blocking layers using metal oxides perform significant, important functions. Hole blocking layers also exhibit important functions when photoelectric conversion elements such as solid dye-sensitized solar cells are used for conversion of weak light such as room light to electricity.
It has been reported that when weak light such as room light is converted to electricity, loss currents due to low internal resistances in the photoelectric conversion elements are conspicuous (see Non-patent document 8). In this regard, it is possible to expect improvement in the photoelectric conversion characteristics by providing hole blocking layers to raise the internal resistances and suppress loss currents. On the other hand, however, rise of the internal resistances makes current extraction difficult. Hence, it is extremely difficult to satisfy both of raising the internal resistances and achieving good photoelectric conversion characteristics.
Therefore, under the current circumstances, none of the photoelectric conversion elements studied so far and including hole blocking layers have been able to obtain satisfactory characteristics in conversion of weak light such as room light to electricity.